The present technology relates to a separator, a battery, a battery pack, an electronic device, an electric vehicle, a power storage device, and a power system. More specifically, the present technology relates to a separator in which a second layer is provided on at least one face of a first layer.
In the past, polyolefin microporous polymer films, such as polypropylene and polyethylene, or multilayer films thereof, have been widely used as a separator. However, these separators suffer from a problem in that air hole clogging and contraction occur due to the heat generated by internal short circuits or overcharging of the battery. When the separator contracts due to the internal heat of the battery, the positive electrode and the negative electrode come into direct contact at the contracted portion, and this contact causes various problems to occur.
Accordingly, recently, to solve these problems with the separator, research has been carried out relating to separators in which a porous film that includes ceramic particles and a binder is provided on a surface. In a separator having such a configuration, heat resistance is conferred to the polyolefin material, so that melting and contraction of the polyolefin material under a high temperature environment of 120° C. or more can be suppressed.
In such a separator, not only is heat resistance conferred to the polyolefin material, but oxidation resistance, mechanical strength and the like can also be conferred. Since these effects are increased the greater the content of the ceramic particles is, in most of the related art, it is said that a range of 80% or more is preferred for the content of the ceramic particles.
However, for vehicle-mounted batteries that have been drawing attention in recent years, such batteries need to achieve a high power density. In order to meet this requirement, various investigations regarding the respective battery parts have been performed. There is a need for a technique to increase the pore size and the porosity in the separator.
To meet this requirement, Patent Literature 1 proposes a technique for increasing the pore size and the porosity by appropriately suppressing solution/solvent replacement and solvent diffusion by, before dipping a polymer solution in a poor solvent bath, absorbing moisture to form a thin porous film on the surface.
Further, to meet the need for even higher battery capacity, alloy-based negative electrodes are being investigated that have silicon (Si) or tin (Sn) as new negative electrode materials. Since such an alloy-based negative electrode tends to expand in volume during charging, there is a need for a separator that can absorb the expansion in volume of the electrode by crumpling.